Acoustic device



une 10, 1930- R. v. l.. HARTLr-:Y 1,762,981

ACOUS TIC DEVICE Filed June 6,l 1928 A TTONEV Patented June 1o, 1930 UNITED STATES.

lPA'PEN'T oFFlcE RALPH 'V'. L, HARTLEY, OF SOUTH ORANGE, NEW JERSEY, ASSIGNOR I O BELL TELE- :PHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION F y NEW YORK ACOUSTIC DEVICE Application led'flune 6, 1928. Serial No. 283,200.

This invention relates to acoustic devices and particularly to an electrostatically operated device. f

An object of this invention is to radiate 5 sound energy with substantially uniform eili-y ciency over at least a large portion of the frequency range employed in speech and muslo.

Another. object of this invention is to' change electrical vibrations into sound viio brations or 'sound vibrations into electrical vibrations through the use of a charged dia.- phragm suspended in an electrostatic field. fI-Ieretoforeattempts have been made to use thin electrically conductive diaphragms having negligible mass and elasticity loosely sup-q.-

tween the metallic members by equal attractive forces on its either side if the diaphragm is caused to approach one of the metallic members by an external force the attractive force of that member towards the diaphragm increases and the diaphragm approachesJst-ilgl nearer the onemember and will come tol-'re tending to hold itin that position depends in a position nearer the one member"'than the other. Obviously a diaphragmsupporb ed between attractive forces in this manner can not be said to be in stable equilibrium. To prevent the diaphragm in such a device from being in unstable equilibrium, external restoring forces are required which necessitate the use of .a stronger and heavier dia- 40 phragm.

This invention overcomes the diiiculty experienced in the method outlined above by placing the electrically charged diaphragm between metallic screens which are electrically charged with the same-polarity as the diaphragm. The diaphragm is then held in stable equilibrium-between the screens under electrostatic forces of repulsion. `If the diaphragm through some outside influence is '59 caused to approach near one of the screens the repulsive force .between the diaphragm" andthe one screen willincrease and on the removal of the external force the diaphragm will be forced by the repulsive force of the nearer screen to the mid position. It is thus seen that the diaphragm according to .this invention is held in stable equilibrium under the restoring forces of electrostatic repulsion, and since no external restoring forces are used, a very thin and light foil may be used for the diaphragm. v

In one embodiment of this invention a thin foil diaphragm is loosely supported between two grid screens, the grid screens, being so constructed with a large number of perforations-that although they extend along the entire surface of the diaphragm a large portion of the diaphragm is not covered by the screens. The grid screens are supported between two plate screens which are perforated to permit thev passa-ge of sound waves.

If, for example, the outer or plate screens 'are given a negative charge, the inner or grid screens are charged positively. Howdue to the incomplete shielding of the y-...the inner screens it also takes up a -Iao'si-tivecharge and so is repelled by both of the inner screens in a manner similar to the repulsion of the leaf of an electroscope. The foil is therefore in stable equilibrium at the mid position.l The restoring force upon the location of the screens, the sizes of their openings and the polarizing battery. For use at low frequencies, particularly, this restoring force should be made as small as possible without sacrificing stability.

When an alternating electrometive force is applied between the two inner screens one attracts the foil land the other. repels, thus setting it in motion at the frequency of the electromotive force. Similarly, motion given to the foil bythe air sets up an electromotive. force between the two inner screens. High impedances' maybe inserted yin the leads to the twoA outer screens and to the foil to prevent induced currents owing. between themnnder the action of the volt-l age applied between the inner screens. To make the force on the foil more uniform the 10 ing the repulsion of the inner screen andattraction of the outer, this arrangement has several disadvantages as compared with the double screen. There would be no protection against the rupture ofthe foil byexcessive air pressure tending to move it away from the.

Yso

screens. The restoring force would not be symmetrical about the position of equilibrium, which would tend to result inv distortion. The double screen arrangement is also analogous to the push-pull amplifier in its elimination of even orders of modulation.

By having the diaphragm, according to this invention, under electrostatic forces of repulsion, no external restoring forces are required to hold the diaphragm at its mid osition between the polarizing plates. o tension is, therefore, placed on the diaphragm and it can he made of very -light and thin material such as foil, having negligible mass and elasticity. By employing a diaphragm having negligible mass and elasticity an electrical transmitter or receiver having high efficiency and uniform response over a Wide range of frequencies is obtained.

Anelectrostatic transmitter arranged according to this invention has a diaphragm so free to move that it will follow faithfully the wave form of the sound giving a velocity response rather than a pressure response, the pressure of the wave front passing through the foil instead of being reiiected back.-`

Referring to thel drawings, Fig. 1 shows an acoustic device constructed according to this invention and is a sectional view along the lines '1 1 of Fig. 2, showing the relative arrangementv of the diaphragm, grids and plates together with the energy supply circuit.

Fig. 2 is an elevation view of a transmitter or receiver, constructed according to this invention, with portions removed to show the details of construction.

Fig. 3 is the equivalentelectrical circuit of the transmitter or receiver showing the accusa tic load connected to the electrical load.

The foil diaphragm 5 is loosely supported between and equidistant from the grids 6. The, grid members 6 are so arranged with respect to each other on either side of the diaphragm 5 that their apertures 7 are offset with respect to each other. This gives a more uniform distribution of the forces exerted by the grids on the diaphragm. Plates 8 are arranged on either side of and equidis- `tant from grids 6. Plates 8 are provided with apertures 9 which permit the passsage of air to and `from the diaphragm '5. Transformer 10 serves as an input transformer to conduct the alternating voltage to the grid 6 when the device is used as a receiver and serves as an output transformer when the device is used as a transmitter. The ends of winding 11 of transformer 10 are connected each'to one of the grids 6. The mld-point of windingll is connected tothe diaphragm 5 and tothe positive side of polarizing batteryv 14. The negative side of battery 14 is connected through choke coils 13 tothe two plates 8'. The two coils 13 are high impedances which serve to prevent induced currents from flowing between the plates and between the plates and the diaphragm under the action of the voltage applied between the completely shield the diaphragm 5 from the plate 8 so that the diaphragm 5 also takes a positive charge. Since the diaphragm 5 and the grids 6 have electrical charges of the same polarity the grids 6 both repel the diaphragm 5 and since the grids are arranged on either side of the diaphragm it is held in stable equilibrium at its mid position between .the grids. An alternating electromotive force applied to the terminals ofwinding l2 of transformer 10 causes, since the grids are connected to the outer ends of winding 11 and the diaphragm is connected to the midpoint of winding 11, the charge on one grid to be increased while the charge on the other 'grid is simultaneously decreased. This causes one grid to attract and the other grid to repel the diaphragm thus setting it in motion at the frequency of the electromotive force. In the same way motion given to the diaphragm 5 by sound waves sets up an electromotive force between the two grids 6.

Fig. 3 shows the equivalent circuit of this invention used as an electrostatic transmitter or receiver.

Re=the electrical load resistance.

C=the capacity of the system to an alterv nating voltage applied to the grids.

T=the equivalent transformer connecting l the acoustic load to the electrical load.

N2 =the equivalent electrical winding 'of the transformer T.

N1=the equivalent acoustic Winding of the transformer T.

m=the mass of the diaphragm.

Rn=the acoustic load resistance.

The ratio is the ratio of the turnsiof 65 tion holds for-approximate purposes when fined as the potential per unit charge on each grid when the plate charges, are zero. By expanding the coefficients by Taylors theorem about the central equilibrium posltion of the membrane and finding the mechanical,

. force in the usual way by differentiation, an

-between two conducting plates. p Maxwells notation, thel radius of the wires Maxwellslrst-treatment of the subject of expression for the force is obtained in terms of one potential coeiicient and its derivatives.

Clerk Maxwell in Vol. 1 of Electricity and Magnetism published by MacMillian Company, London, has given a treatment of the electric field due to a charged grid of wires Following may be denoted by c, their distance apart by a, the distance from the grid to the moving membrane by b1 and the distance from the grid to the stationary plate by b2. The p0- tential coeiiicient giving the potential on the grid due to unit charge on that grid is denoted by pn.

It isshown that to make the second harmonic distortion low, it is desirable to make the second derivative of p11 with respect to 35 'the dis/placement equal to Zero at the equilib- -rium point. This condition kwill also makethe lelectrostatic elasticity zero. The requirement to fulill thisv v condition is the'charged grid-required b2 to be large compared with a. He later? gave a Method of Approximation` in the form of a series which gives good results-when b1 and b2 are of they same size or somewhat smallerthan a. In

` order to express the results yof this investigaassumed that y,imi'in a reasonably'simple form, it will be where the b may either b1 or b2. This rela- 4b a (2)l .colabining this relation with Equation (i) 4b a 15 (3) Equation (1) then gives'the condition for minimum distortion and electrostatic elasticity and Equation (3) -gives the conditions under which the approximations hold.

. Representing the alternating charge on the grids by g and the steady applied voltage on the plates by V, the expression for the lforce on the membrane may be written p F (2b2 .11a/) b2+b1(b2-.11ct)` If 51:62:@ the expression reduces to 2. 1,) The capacity of the system to the lalternating charge g is where A is the area of the membrane. If 'a be the alternating voltage applied between the two grids then K' is dened by (6). It represented in the equivalent circuit by the turnsratio l of the transformer T. e

In order that the electrostatic element be a highly eiiicient device, two thingsare necessary; first that Z, the mechanical impedance per unit area ofithe diaphragm with its air v load, primarily the radiation resistance to the air, and second that the reactance of the ance It the membrane be made of .0005" aluminum,hits mass reactance will bev only about twice the radiation resistance-of the air at 5,000 cycles which would not introv capacity C be large compared to the impedduce a large loss. The second requirement is much more serious,'however. Withpracticable voltages and separations, the capacity C. will probably always predominate, which, for a good frequency response characteristic, would lead to an inefficient arrangement.

In' order to obviate the last difficulty a negative capacity as secured by one of the various vacuum tube circuits'may be connected across the terminals of thestructure.

A relatively small corrective capacity would be required. f

In considering the arrangement as a transmitter, b the distance between grid and'plate istaken as equal to the -distance between grid 4and diaphragm, a the separation between the .wires of the grid and V the steady voltage applied to the plates.- The choice of thevalues of a and bare restricted and iixthe value of c, the radius of the grid wires, in the manner previously discussed.

The insertion of numerical values in the formulas will show that the motional impedance of the electrical elements on the mechanical side will be negligible for practicable value of'polarizing voltage. The diaphragm may then be considered simply as a membrane suspended in the sound path. Let the velocity of the membrane be w', the current then at the left of the transformer invFig. 3 of the drawing is given by a (1 -.113)

If the resistance Re is made equal to the reactance of the condenser C at some frequency f6, then below this frequency the current in the resistance Re will be substantially the same as z'. We have then that 411(2 .113) -Afo i and thevoltage across the resistance Re due to the velocity :i: of the membrane is Assuming values for the consonants of f0'=5000 cycles a=.25 cms.?.0985 b .10 cms.= .0394 `0=.0065 cmS.=.0026

movefreely.in the sound field without intro-- ducin'g distortion, vvhere p is the pressure of the sound wave, and--(--z-Tls The quantity Y @X10B into the expression for voltage sensitivity. It" does, however, determine the value`of the resistance Re, the proper value may readily micro-microfa'rads. For

megohms and the capacity 0= as rectangular in shape, it will be understood that the plates ,and diaphragm may -be of other shapes, circular, for example, and that said plates and diaphragm may be held in spaced relationship in any desiredmanner.

What is claimed is:

An acoustic device comprising a metallic diaphragm, a perforated plate member on each'side of said diaphragm, a perforated grid member between each of said plate memers and said diaphragm, a direct current sourcefconnect'ed at its one pole to said diaphragm and at its opposite pole to said plate members, and means for applying a'varying electrical force .to said grid members, the perforations in saidgrid and plate members being offset with respect to each .other whereby an electrical charge of one polarity on said plate members may cause a charge of opposite polarity on said grids and through the perforations in said grid member on said diaphragm.

In witness whereof, I hereunto ,subscribev my name this 5th day of June, 1928.

RALPH V. L. HARTLEY. 

